Book excerpt: Tables of cosmic-ray cutoff rigidities using the trajectory-tracing technique are given for five Epochs of the geomagnetic field. These values have been determined utilizing the International Geomagnetic Reference Field with time derivatives applied so that the coefficients for the field model are appropriate for the following Epochs: 1955.0, 1965.0, 1966.5, 1970.0, and 1975.0. Each table includes the geographic coordinates and L value of each location together with the main cutoff rigidity, the Stormer cutoff rigidity, and the effective cutoff rigidity. Tables for both vertical and non-vertical directions are included, as well as a listing of the FORTRAN computer program used to calculate these values. (Author).